1. Field
Example embodiments of the present patent application generally relate to a sheet conveying device and an image forming apparatus including the sheet conveying device. More particularly, the present patent application relates to a sheet conveying device conveying a sheet-type recording medium between a drive roller and a driven roller contacting the drive roller with pressure, and an image forming apparatus including the sheet conveying device therein.
2. Discussion of the Related Art
Electrophotographic image forming apparatuses rotate a drum-shaped or belt-shaped photoconductor to charge a surface thereof and optically write an electrostatic latent image on the surface. Toner is supplied to develop the electrostatic latent image to a toner image, which is transferred onto a recording medium directly from the photoconductor or via a belt-shaped intermediate transfer member to print the toner image on the recording medium.
Color image forming apparatuses include one or more photoconductors to sequentially transfer respective single color toner images formed thereon onto an intermediate transfer member to form a composite color image, which is then transferred onto a recording medium.
Image forming apparatuses employing an intermediate transfer member can convey various types of recording media, such as thin papers, thick papers, postcards, and envelopes, thereby providing a wide range of recording medium transferability. Generally, a drum-shaped intermediate transfer member or a belt-shaped intermediate transfer member is most widely used.
In general, the intermediate transfer member is driven at a constant speed through various areas and units for image forming, including a primary transfer area, in which respective single color toner images are sequentially transferred onto an intermediate transfer member to form a composite toner image, and a secondary transfer area in which the composite toner image is transferred from the intermediate transfer member onto a recording medium. However, when a recording medium having a certain critical thickness enters the secondary transfer area, the speed of the intermediate transfer member may decrease for a short period of time. Accordingly, an intermediate transfer unit including the intermediate transfer member may transmit a brief decrease in speed caused in the secondary transfer area and cause deviations or defects in images in the primary transfer area.
In addition, a trend toward downsizing color image forming apparatuses causes a secondary transfer unit located at the secondary transfer area and a fixing unit located at a fixing area to be disposed close to each other, so that a secondary transfer operation and a fixing operation can be conducted simultaneously. In such a configuration, however, the fixing unit suffers from the same drawback as the intermediate transfer unit. Specifically, when a sheet-type recording medium having a certain thickness enters the secondary transfer area, a fixing roller or belt driven at a constant speed in the fixing unit may experience a brief decrease in speed, thereby transmitting the brief decrease via the fixing unit and causing deviations or defects in images in the secondary transfer area.
In short, a thick sheet-type recording medium causes a short-term decrease in speed of an intermediate transfer drum, an intermediate transfer belt, a fixing roller, and/or a fixing belt when the sheet-type recording medium enters a secondary transfer area or a fixing area.
To eliminate the above-described drawbacks, a feed-forward control may be conducted to maintain a belt member moving at a constant speed. In feed-forward control, a speed of a drive source driving a belt member formed in a closed loop is controlled at a preset, reference timing, in a preset, reference amount, and for a preset, reference period of time so as to correct any decrease in speed of the belt member when such a thick paper enters a transfer unit.
However, changes in the amount of such speed control are abrupt, and can be modeled as rectangular waveforms.
A different sort of control may also be conducted to reduce a decrease in the speed of the fixing roller when a thick sheet of paper enters the fixing unit. In this control, a thickness of a sheet-type recording medium is detected in advance and a force applied to a pressure roller of the fixing unit is adjusted according to the detected thickness.
However, such control requires an additional mechanism to adjust the pressing force, which causes an increase in costs. In addition, it is mechanically difficult to accurately control the pressing force.